Solid waste materials of the type typified by municipal waste have traditionally presented problems of disposal which have become increasingly critical in recent years as a result of not only a rapidly increasing population but the compounding difficulty of a drastic increase in per capita production of solid waste. Conventionally, such solid waste has been disposed of by such means as incineration and landfill. Obviously, with the ever increasing concern with problems of natural resources and the dwindling supply of acreage suitable for landfill operations within a reasonable distance of population centers, both of these methods of solid waste disposal are becoming less acceptable.
At the same time that both population and the per capita production of refuse have been increasing, the national energy shortage has made it important to give full consideration to refuse as a potential source of energy. Based on recent (Spring 1973) figures from the National Center for Resource Recovery, municipal solid waste contains a total of about 50% organic matter, in accordance with the following table:
TABLE 1 ______________________________________ COMPOSITION OF MUNICIPAL SOLID WASTE COMPONENT DRY BASIS ______________________________________ Glass 9.0% Ferrous Metal 7.0% Aluminum 0.7% Other Nonferrous 0.3% Paper 32.0% Textiles 1.8% Rubber 1.0% Plastics 1.5% Other Organic 13.7% Other Inorganic 8.0% Water 25.0% TOTAL 100.0% ______________________________________
The organic fraction of municipal solid waste is an important source of energy, which has an average heat value of about 8500 Btu per dry pound and an annual potential for the United States equivalent to nine billion gallons of No. 2 fuel oil. On a dry basis, its heat equivalent is about two-thirds that of Ohio coal, while its sulphur content is nil compared to low sulphur coal. Most important, it is a renewable source of energy which is readily available on a year-round basis in energy dependent urban areas.
In order to realize the potential of municipal refuse as fuel, it must be converted to a form of sufficiently uniform physical and combustion characteristics to suit the specific requirements of the furnace or other burner in which it is to be used. From this standpoint, it is essential that the inorganic constituents be removed as completely and efficiently as possible from the organic fraction, so that all of the organic material can be processed to fuel while the ash content is minimized. A less obvious requirement is that there must also be complete separation of organic material from the removed inorganic fraction, since any organics retained with inorganics which ultimately reach land fill constitutute a putrescible nuisance and potential hazard.
Except for the present invention and my prior work disclosed in my above U.S. Pat. No. 3,830,636, attempts to produce solid waste fuel on a practical purpose have been primarily directed toward better separation techniques and reduction of particle size in dry treatment systems. This approach, however, is complicated by a number of factors. For example, solid waste is heterogeneous by nature, but it is subject to daily and seasonal variations in moisture content which can range from as low as 15% to more than 50%, resulting in variable heat content on any unit weight basis. For example, statistics on dry shredded fuel characteristics released in 1974 by Midwest Research Institute show an organic content ranging from 24.5% to 66.9% with an average of 55.3%, an ash content ranging from 7.6% to 19.5% and an average of 14.8%, and a water content ranging from 14.3% to 66.3% with an average of 29.9%. The Btu value ranged from 2293 to 6446 per pound and averaged 5086.
Most recent attempts to improve solid waste fuel by a dry process approach have utilized combinations of shredding apparatus with air classification to provide a fuel for immediate or near term firing on or near the site. The loose bulk density of solid waste fuel produced in this manner may have a range as wide as 5-12 pounds per cubic feet on an oven dry basis, as compared to 50-55 pounds for coal. The resulting bulk of dry solid waste fuel makes it uneconomical to transport any significant distance or to store in any substantial quantity; storage is further complicated by its tendency to deteriorate rapidly under favorable storage conditions and because such fuel cannot be exposed to weather.